AFRL Backs New Type Of Combat-Aircraft Engine

It has been almost three decades since the U.S. last set out to develop an all-new combat-aircraft engine, but more than 50 years since the turbojet gave way to the turbofan. Now the U.S. is embarking on development of a new generation of fighter engine with an architecture it considers as fundamental an advance as the turbofan was over the turbojet.

Being part of a research effort that could produce the dominant combat-aircraft engine of coming decades is critical for industry. So General Electric and Pratt & Whitney are breathing easier after being selected by the U.S. Air Force Research Laboratory (AFRL) for the Adaptive Engine Technology Development (AETD) program to mature fuel-efficient, high-thrust powerplants for post-2020 upgrades to the Lockheed Martin F-35 and future “sixth-generation” combat aircraft.

AETD is a follow-on to AFRL's $524 million Adaptive Versatile Engine Technology (Advent) program under which GE Aviation and Rolls-Royce North American Technologies will demonstrate engines in 2013. Selection for AETD is a coup for Pratt, which in 2007 lost out to GE and Rolls in the Advent competition, but a blow for Rolls, which did not receive the nod for the follow-on program.

“When we were not selected for Advent, we did a lot of our own rig work,” says Jim Reed, Pratt & Whitney's director of advanced engine programs. “But that only mattered if we were selected for AETD. We had to build a strong proposal.” At stake is the potential development of fuel-efficient engines to upgrade the F-35 after 2020 and to power future Air Force and Navy air-dominance fighters that could enter service around 2030.

The last time the U.S. embarked on development of an all-new combat engine was in the early 1980s with the launch of the Joint Technology Demonstrator Engine (JTDE) program, which led to Pratt & Whitney powering both the Lockheed Martin F-22 with the F119 and the F-35 with a further development of that engine, the F135.

Having succeeded in killing the GE/Rolls-Royce F136 alternative engine for the F-35, Pratt's supporters in Congress threatened to cut funding for AETD, fearing it was a backdoor maneuver to a competitive engine. But Air Force reassurances that its goal is to mature technology and not to develop an engine—coupled with the selection of Pratt over Rolls—should defuse criticism of AETD.

“We will not produce a prototype engine, but run core engines,” says Tim Lewis of AFRL's Propulsion Directorate. “All three proposals were technically acceptable, but funding drove us to select two of the three,” he says. Rolls states that it is “disappointed by this decision but . . . continues to work with the Air Force Research Laboratory on important programs such as Advent and HEETE [Highly Energy-Efficient Turbine Engine].”

Where Advent is demonstrating high-pressure-ratio core and adaptive-fan, variable-bypass, low-pressure system technology to reduce combat-engine specific fuel consumption (SFC) by 25%, AETD will fully mature adaptive engines for possible early entry into engineering and manufacturing development (EMD) after 2020. As with Advent, there will be “significant” cost sharing by the contractors, says Lewis.

Advent and AETD are developing and maturing technology for engines with a “three stream” architecture. In addition to the high-pressure core and lower-pressure bypass streams of a conventional turbofan there is a third, outer flowpath that can be opened and closed. For takeoff, the third stream is closed off to reduce the bypass ratio and route more of the airflow through the core to increase thrust. In cruise, the third stream is opened to increase the bypass ratio and reduce fuel consumption.

The third stream can cool the cooling air used for thermal management of the engine hot section, the fuel used as a heat sink for aircraft systems, and the walls of the augmentor and nozzle. The architecture can also reduce aircraft drag. Inlets are sized for maximum airflow on takeoff, but capture more air than the engine needs in cruise, resulting in spillage. The third stream can bypass the extra air, reducing spillage drag, and the additional flow can be used to fill in the aircraft boat-tail, reducing base drag.

AFRL calculates adaptive technology will improve engine fuel efficiency by 25% over the F135 powering the F-35, increasing aircraft combat radius by 25-30% and persistence by 30-40%. The engine could also help address the anti-access/area-denial challenges posed by a potential conflict with an near-peer adversary such as China, says AFRL. This could be achieved via increasing supersonic-cruise radius by 50% and reducing the aerial-refueling tanker burden by 30-74%.

Under the 48-month AETD program, GE and Pratt will design engines with 25% lower SFC, but 5% more military (dry) power and 10% higher maximum (reheat) thrust than the F135. “We will take that engine through preliminary design review,” says Reed. The engine must be sized to fit the F-35 with “only modest modifications,” he says.

AETD will take adaptive engines to “robust” technology and manufacturing readiness levels (TRL/MRL 6) in wait for a future EMD program, says Lewis. “Engines lead airframe development by several years and we need to be ready if and when they launch the next aircraft EMD,” he says.

“The time is now for sixth gen,” Reed says. “We may be 15 years away from needing an engine, but we have to start reducing risk now, because come 2020 we will need to begin an EMD program.”

Manufacturers expect future fighters to require more thrust, better fuel economy and thermal management, and more electrical power generation. “But we don't know exactly what the transformational requirement will be for sixth gen, says Reed. “Adapatability may be the missing piece.”

Phase 1 of the AETD program, which runs through mid-fiscal 2015, includes preliminary design of the engine and testing of annular-combustor and high-pressure compressor rigs. Phase 2, which will conclude in fiscal 2016, consists of fan-rig testing and an engine core test, allowing for a notional first full engine test as early as 2017.

“There will be additional focus on augmentor and exhaust technology that is not part of Advent,” says Lewis. This will include developing an exhaust system that enables the low-pressure third stream to mix effectively with the higher-velocity core and bypass flows. “We will integrate an exhaust that produces thrust from the third stream,” he says.

In parallel, studies with the airframers will identify potential applications of the common core beyond 2020. “We will conduct quantitative assessments of AETD-derived propulsion systems,” says AFRL's Lewis.

GE, meanwhile, will begin high-pressure core tests this week under the Advent program. Rolls-Royce's LibertyWorks will begin testing its core in late November/early December, says Matt Meininger, AFRL's Advent program manager. GE's full engine is “75%-plus” complete and will go to test in July 2013, he says, while Rolls's engine is 90% complete and scheduled to begin tests in October next year. Pratt will deliver an adaptive-fan test rig—developed largely with company funds—to AFRL in February/March “to jump-start us [on AETD],” says Reed. Although Pratt lost the Advent competition, Meininger says AFRL was able to come up with more funding to help it modify an existing rig by adding a third stream to a two-stage fan, putting the company on a “fast track” to compete for AETD.

For Rolls, there was some consolation this week when it began tests of a high-pressure-ratio compressor demonstrator under AFRL's HEETE program, which aims to demonstrate technology to reduce specific fuel consumption by 35% for embedded engines in future subsonic transports and unmanned aircraft. GE has also delivered a HEETE compressor rig to AFRL, with testing expected next year.

Demonstration of a high-pressure core—compressor, combustor and turbine—is planned for 2016 under HEETE, with tests of a highly efficient turbine engine planned for 2019. Reducing SFC by 35% would increase transport-aircraft range by 50% and payload by 25%, and double the loiter time of intelligence, surveillance and reconnaissance UAVs, estimates AFRL.

Although funds are not available, the Air Force Research Lab would like to find a way to keep Rolls working on adaptive engine technology. “Each of the three companies has a unique architecture concept for the third stream,” says Jeff Stricker, AFRL Propulsion Directorate chief engineer. “They have three approaches to adaptive fans and how to vary the third stream with the two main flows.”

“We believe three-stream is the architecture for the next generation of aircraft,” says Lewis. “Of course we would like to keep all three approaches available to us.”